AST 248 HW for Chapter 6
Ryan Richards
Review Questions
What are the three lines of fossil evidence that point to an early origin of life on Earth?
Discuss each line and what it tells us about when life arose. What are the implications of an
early origin for the possibility of life elsewhere?
1) Stromatolites: Rocks characterized by a distinct layered structure. The book mentions that
the ancient stromatolites look similar to the microbes today. The microbes on the top later
of these rocks are photosynthetic. Their structural similarities imply that they are fossil
remnants of early life.
2) Microfossils: These are fossilized cells. The oldest fossils have great controversy since
they have undergone geological processes over time. Therefore it is difficult to
differentiate what are cells and what aren’t. The younger microfossils have been shown to
contain molecules like hydrocarbons. They can conclude that there was life before about
3.0 billion years ago.
3) Isotopic Evidence: By observing the amount of carbon-12 and carbon-13, it has been
shown that fossils of living organisms have a slightly lower fraction of carbon-13 in them
than inorganic material do. Also the rocks should have been sedimentary before
undergoing transformations. Life also affects the isotopic ratios of other elements such as
sulfur, iron and nitrogen.
How do DNA sequences allow us to reconstruct the evolutionary history of life? What
living organisms appear to be most closely related to the common ancestor of all present
life?
DNA sequences allow us to reconstruct the evolutionary history of life. By looking and
comparing DNA sequences, we can observe similarities which imply that two organisms came
from the same common ancestor. The closest relatives of the universal ancestor of all life are the
simple single-cell organisms.
What was the Miller-Urey experiment, and how did it work? Why is the relevance now
subject to scientific debate? How else might earth have obtained the organic building
blocks of life?
The Miller-Urey experiment was an experiment testing the idea that the earth’s early atmosphere
was oxygen free. It suggested that the chemical reactions fueled by sunlight could have led to the
creation of organic molecules. They used small glass flask to simulate chemical conditions of the
early Earth. One was partially filled with water to represent the sea and heated to produce vapor.
They added ammonia and methane gas to the vapor which modeled the atmosphere. The gases
were transported to a second flask, where electric sparks produced chemical reactions. The gas
was then cooled for condensation and the resulting liquid turned brown and chemical analysis
showed there were amino acids and other organic molecules.
The relevance is subject to debate since ammonia and methane were not present in the Earth’s
early atmosphere. Scientists believe that hydrogen can play a role in facilitating the development
of organic molecules. It is unknown whether hydrogen was present in the early atmosphere.
The other sources of organic building blocks are from reactions near deep sea vents and material
from space such as meteorites.
What do we mean by an “RNA world”, and why do scientists suggest that such a world
preceded the current “DNA world”?
An RNA world is a world where genetic information is stored in RNA instead of DNA.
Scientists believe this preceded the current DNA world since RNA is a simpler molecule than
DNA. For example, RNA is single stranded and DNA is double stranded with a backbone
structure. Some may know from biology how complicated it is to make DNA.
Briefly summarize current ideas about the sequence of events through which life may have
originated on Earth. What role(s) might clay or other inorganic materials have played?
1) The formation of amino acids and other organic molecules
2) Larger organic molecules like RNA formed from smaller organic molecules. These
reactions could have taken place through inorganic molecules like clay. Pre-cells enclose
RNA strands
3) Self-replicating RNA forms inside pre-cells and natural selection takes over.
4) Natural selection continues until complex organism form.
5) DNA evolves from RNA and dominates. DNA becomes the basic unit of encoding
genetic information.
Briefly describe the possibility that life migrated to Earth. Also discuss the possibility that
Earth life might have migrated to other worlds, and the implications of migration to the
search for life elsewhere.
Due to the impacts of meteorites, surviving microbes on these meteorites can travel to another
planet after it had hit the Earth. We can deduce that microbes from the Earth may have traveled
to nearby planets. However, it would be very difficult to determine whether or not life arose from
the Earth or from a nearby planet.
Why do we think that evolution would have proceeded rapidly at first, and what fossil
evidence supports this conclusion?
The reason is because the simple organisms back then reproduce very quickly. Also the lack of
mutations made things simpler as well. The fossil evidence supporting this conclusion came from
photosynthetic organisms. This implies that photosynthesis occurred rapidly if there was rapid
evolution.
How do we think that eukaryotes evolved? What time constraints can we place on when
eukaryotes first got cell nuclei?
We think that eukaryotes evolved by bacteria engulfing one another. For example, this might
explain why mitochondria has DNA as well (its circular like we find in prokaryotes). The two
bacteria form a symbiotic relationship (simply they help each other out) which eventually causes
them to form into one organism. There are fossils that go back 2.1 billion years showing that
eukaryotes had cell nuclei.
What was the Cambrian explosion? Briefly discuss ideas about what might have caused it
and why no similar event has happened since.
The Cambrian explosion occurred between 540 and 500 million years ago. During this period we
began to see a diversification of life on Earth. Although there is no definitive answer to the
cause, there are some key ideas on what may have caused it. These factors include the presence
of oxygen, the evolution of genetic complexity, climate change and the absence of predators. The
last of these ideas is why we haven’t seen a period like the Cambrian since.
Summarize the history of oxygen buildup as it is understood today, and describe key
mysteries that still remain. When did oxygen reach current levels?
There was an assumption made that cyanobacteria began producing oxygen at 2.7 billion years
ago. There was also a hypothesis that rock and ocean minerals oxidized which removed some of
the oxygen in the atmosphere as rapidly as the cyanobacteria produced it. After the oxidation
could the atmosphere build up. The concentration of oxygen began to increase about 2.35 billion
years ago which would then be able to support complex life. Oxygen reached its current level
about 1 billion years ago.
The mystery comes from the fact that it a long time for the oxygen to build up in the atmosphere
possibly due to the oxidation in rocks and ocean materials as mentioned aboved.
Would You Believe It
We discover evidence of life, in the form of a particular ratio of carbon-12 to carbon-13 in
rock that was originally formed in sediments and is 3.9 billion years old.
This one is interesting. It is possible however difficult to say since the sediments can’t be date for
rocks older than 3.85 years old as mentioned in the book.
We discover an intact fossil of a eukaryotic cell, with a cell nucleus that is 3.0 billion years
old.
This is possible considering the earliest and oldest fossils of eukaryotic cells have been shown to
have nuclei as far as 2.1 billion years ago. The book also mentions that these cells could have
had cell nuclei earlier but they haven’t been able to recognize them in their fossil records.
We discover a preserved, 3.5-billion-year-old microfossil that apparently had a genome
genetically just like that of many modern animals.
This is an interesting one. One can argue for this or against this. There has been evidence of early
life in microfossils.
We discover clear evidence that life arose on a high mountaintop, not in the oceans.
I don’t believe this. Early life of mountaintops were not suitable for life.
We discover a fossil of a large dinosaur that lived approximately 750 million years ago.
This is unlikely. The mass extinctions would have eradicated any fossils. Also dinosaurs arose
later than that. The first animals however did arise around this period.
We discover that, contrary to present belief, oxygen was abundant in Earth’s atmosphere
at the time when life arose.
This is not believable. The book says oxygen would break up organic bonds. Therefore, oxygen
would have been detrimental to early life on Earth.
We discover a crater from the impact of a 10-kilometer asteroid that dates to about 2500
years ago.
This is unlikely since an asteroid of that size would have destroyed the Earth.
We discover an asteroid about 3 kilometers across that is on a collision course with Earth.
This is believable.
We find fossil remains of an early primate that lived about 50 million years ago and was,
from all appearance, identical to a modern gorilla.
This is not believable. The earliest primates came before this time and there an extremely small
chance that they looked like gorilla. Also gorillas diverged from monkeys much later 50 million
years ago.
The first life created in the laboratory has an RNA genome, rather than a DNA genome.
This is plausible considering early life consisted of RNA. RNA is simpler than DNA and
therefore much easier to recreate in a lab setting.
Quick Quiz
Which statement about Earth’s ozone is true? (a) It formed only after the atmosphere became
rich in oxygen.
The hypothesis that an impact killed the dinosaurs seems (a) well supported by geological
evidence
According to the fossil evidence, modern humans (b) evolved on a lineage that split from
other apes 6 million years ago or more
Quantitative Problems
Bacterial Evolution. Suppose that a mutation occurs in about 1 of every 1 million bacterial
cells, and suppose that you a bacterial colony in a bottle like that described in Cosmic
Calculations 6.1 (in which the bacteria divide every minute). Given the number of bacteria
in the bottle after 1 hour, approximately how many bacteria would have some type of
mutation? What does this tell you about why bacteria often evolve resistance to new drugs?
We begin by considering only one bacterium in the bottle. Then after one minute we have two
bacteria. After two minutes we have four and after three minutes we have eight. The evident
generalization of this
2t,
where t is the number of minutes. After one hour or 60 minutes, there are 1 million trillion
bacteria or 1 x 1018 (which is given in Cosmic Calculations 6.1).
We know that 1 out of every million bacteria have mutations; therefore the ratio is
1/106 or 10-6.
We take this ratio and multiply it by the number of bacteria to get the number of bacteria with
mutations. The answer is 1012 bacteria. From this answer we see that there are more bacteria with
mutations than there are without. As a result, we see that this large number can have different
mutations which would help bacteria resist new drugs.
Please note, the answer is not three bacteria like some may have thought, remember we are
subtracting exponents.